1. Field of the Invention
The field of the invention relates generally to air-fuel ratio sensor diagnosis.
2. Background of the Invention
There are various methods used for determining whether an exhaust air-fuel ratio sensor has been degraded. For example, one method is to look for switching during closed loop control around stoichiometry. If the sensor switching reduced, then degradation could be found. Another method is to determine whether the sensor is operating within predetermined range around a desired air/fuel ratio.
The inventors herein have recognized a problem with prior approaches. In particular, when using a system in which the sensor is exposed to air pumped through a cylinder, the sensor may stop switching, or be forced beyond the predetermined range. Thus, even for a sensor that has not degraded, a false indication can be provided that the sensor is degraded.
The above disadvantages are overcome by a system, comprising: an engine having a first group of cylinders and a second group of cylinders; a sensor coupled at least to said first group of cylinders in an engine exhaust; a controller for operating said first group with air and substantially no injected fuel, operating said second group with air and injected fuel, reading an output of said sensor, and determining degradation of said sensor based on said reading.
By determining whether the sensor indicates lean when exposed to air pumped through a cylinder that is not combusting, it is possible to check functionality of the sensor and prevent false indications that the sensor has degraded.